The present invention relates generally to image capture devices that have a photo sensor or sensors that have dual resolutions. The photo sensor or sensor array has a set of high-resolution photo elements and a set of lower resolution photo elements. Substituting the low resolution signal for the high resolution signal in areas of the image that have low signal strength and low spatial frequency can improve the signal to noise ratio of the image.
Digital scanners typically create an image by focusing an area of a page onto a photo sensor. The photo sensor is typically a charged-coupled device (CCD). The CCD is typically composed of a row of photosensitive elements. The row of elements is typically imaged across the width of the scanner bed. The width across the bed of a scanner is typically called the x-direction and the length along the long axis of a scanner bed is typically called the y-direction. To create an image of the page, the row of photo sensors is typically moved along the y-direction, taking exposures corresponding to each row along the x-direction. The optical resolution of the scanner in the x-direction is equal to the number of photosensitive elements in the CCD divided by the width of the page. The resolution of a scanner in the y-direction is a function of the movement rate of the CCD along the y-direction and the exposure rate of the CCD.
Today one of the main purchasing criteria is the optical resolution of a scanner. This tends to drive scanner manufactures to increase the optical resolution of scanners. For a given CCD die size, as the optical resolution in the x-direction increases, the size of the photo sensor elements decreases. The CCD die size is contained to keep the price of the CCD reasonable. Typically the CCD has three rows of photosensitive elements, one each for detecting red, green, and blue light. The spacing between the three rows of photosensitive elements is one factor in determining the amount of memory needed in the scanner. The larger the line spacing between rows, the more memory needed. Unfortunately the shift registers also need to be in-between the rows of photosensitive elements. The spacing between the rows, and the number of elements in each row, limit the size of each shift register. The smaller the size of the shift register, the lower the signal carrying capacity of those shift registers, which results in a lower possible the signal-to-noise ratio. FIG. 1 shows a plot of the signal-to-noise ratio vs. absolute signal level for a lower resolution CCD and a higher resolution CCD which have the same die size and line spacing. The increase in noise is a bigger problem in the dark areas of an image. This is true for two reasons. First when the image signal is low the noise sources are large by comparison, which negatively impacts the signal-to-noise ratio in the dark areas of the image. Second the human visual system is more sensitive to noise in dark areas of an image.
One of the solutions to this problem is a CCD that contains two different resolutions. The CCD has a row of high-resolution photo sensors and a row of lower resolution photo sensors. The high-resolution row of photo sensors is typically two times the resolution of the lower resolution photo sensors. Color scanners typically have three rows of photo sensors, one row for each color (red, green and blue). The dual resolution color CCD has three rows of photo sensors for each resolution for a total of six rows of photo sensor elements (see FIG. 2). The scanner uses the high resolution CCD for high-resolution scans and the low resolution CCD for low resolution scans. Unfortunately this only solves the noise problem for the lower resolution scans when the scanner is using the low-resolution, higher signal-to-noise photo sensors. Therefore there is a need for a system that can combine the improved signal-to-noise characteristics of the low-resolution sensor signal, with the high-resolution signal, without losing the high spatial frequency information from the high-resolution signal.
A method of combining the low resolution, higher signal-to-noise information with the high resolution, low signal-to-noise information created by a dual resolution CCD.